The enrichment and capture of target cells, for example, cancer cells, bacteria, viruses and other pathogens, from peripheral blood are of great importance for clinical disease diagnosis, therapy monitoring, biology study, and drug development.[1-3]
Circulating tumor cells (CTCs) are cancer cells spontaneously circulating in the peripheral blood or spreading iatrogenic into blood vessels, which is an early step in the cascade of events leading to metastasis.[4] Capture of CTCs provide a potentially accessible source for detection, characterization, and monitoring of cancers. However, isolation of these cells is a significant technological challenge due to their rare numbers and their low recovery rate following traditional batch purification techniques. There are a variety of conventional technologies, such as immuno-magnetic enrichment, flow cytometric cell sorting or polymerase chain reactions (PCR)-based methodology,[2, 5-8] but these are multistep technologies which are complex and have insufficient capture efficiency.
Recent microfluidic CTC devices using monovalent capture agents, including antibodies[9-11] and nucleic acid aptamers[12-13] represent a promising approach to capture cancer cells with highly efficient processing of complex cellular fluids,[14-15] greater simplicity, sensitivity[16-17] and throughput.[10, 18-19] However, monovalent adhesion ligands have difficulties in capturing large-sized entities, such as cells under high shear stress; a flow rate with low shear stress will render the assay time-consuming to process large volumes of blood samples. Thus, most of those devices for CTC capture are designed with complex channel topologies including microposts,[9] herringbone grooves,[20] or 3D structures.[21] Even with these geometries, efficient cell separation requires low shear stress conditions to maximize cell-surface contact.[22]
The cell surface has certain structures that cause higher contact chances than a flat surface,[23] as evidenced from enhanced local topographic interactions between the rough substrates and nanoscale cellular surface components.[24] Both aptamer and antibody have strong non-covalent interactions with cell surface binding sites, for example, cell surface receptors and other biomolecules. However, aptamers are much smaller with molecular weights of about 8-15 kDa, compared to antibodies with molecular weights of 150 kDa.[25] Due to the shape of a cell and a surface structure coated with nanoscale microvilli and filopodia,[26] the combination of aptamers and antibodies with different molecular size can increase the accessibility of biomolecules on the surface of cells and facilitate interactions between cell surface biomolecules and capture agents. This would permit cell capture under high flow rates. Therefore, a combination of aptamers and antibodies can provide for the capture of target cells by binding to cell surface binding sites in a cooperative manner, leading to higher cell capture efficiency (FIGS. 1a and 1b) and solve the need for efficient target cell (e.g., CTC) capture at high flow rate and high shear stress.